July 8, 2011

Worms Fight Parasites With Sex

According to researchers, sex gives worms the power to fight off parasites.

The researchers found that worms forced to reproduce asexually succumbed to a bacterial infection and died.

The team said the results are the most convincing evidence to date for a key theory in evolutionary biology.

The theory said that sex evolved because it allows organisms to reshuffle their genes into new combinations to stay a step ahead of parasites.

The team said that reproducing asexually means there is no need for an organism to search and seduce a mate, fight off competitors, or risk contracting a sexually transmitted disease.

Also, an organism is likely to have a first-rate set of genes under its belt given that it has survived long enough to reproduce.

Many biologists believe that the answer lies within parasites.

The researchers said the genetics "arms race" between a parasite and its host is often referred to as an example of Red Queen-style interaction.

Biologists say that organisms are more likely to reproduce sexually when there are more parasites loping around in their vicinity.

The researchers at the University of Indiana used the round worm Caenorhabditis elegans to do this.

The team engineered two types of worms:  some that could only reproduce by having sex, and others that could only clone themselves.

The team observed the worms consume the bacterium Serratia marcescens, which invades the worms' gust and from there multiplies into every crevice of their body, killing the worms from the inside.

Worms across five different populations that reproduced sexually faired well throughout 20 generations, while all animals that cloned themselves died quickly.

Agrawal said the experiment was "elegant" because it allowed the researchers to demonstrate that it was not simply the presence of the parasite that spelled the end for the cloners, but the presence of a parasite that had co-evolved alongside the worms.

The team created two treatments: one used bacteria from an original stock kept in the freezer, and the other used bacteria that had lived alongside the worms for many generations.

Lead author Levi Morran, an evolutionary biologist from the University of Indiana, told BBC that "the bacteria got more and more infective, but the [clonal worms] did not get more and more resistant, and that is why they went extinct."

"I am really excited about this; I think this is really cool."

"Whether this is actually happening in nature is another thing; we can't know that from a lab system," he told BBC.

He said that it is important to demonstrate under conditions where you expect sex to alleviate the effects of parasites.


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